The present invention generally relates to the field of gas turbine engines. In particular, the present invention relates to an abradable seal for a gas turbine engine.
Abradable seals are often used in gas turbine engines to assist in reducing the operating clearances between surfaces with relative motion. For example, abradable seals may be used in gas turbine engines to help improve the efficiency of the engine and to increase its stall margin. The abradable seal is typically positioned between a stationary component opposite a rotating component. For example, the stationary component may be an outer engine casing or a shroud and the rotating component may be a blade tip, a sealing ring, a knife-edge seal, and the like. In operation, the blade initially engages the abradable seal and rubs or cuts into the abradable seal. The abradable seal helps ensure that the blade tip does not contact the outer casing, it is the abradable material of the seal that is removed, rather than the blade tip. The abradable seal thus reduces the clearance between the stationary component and the rotating component and prevents damage to components of gas turbine engines during rubs. Proper sealing between the abradable seal and the rotating component may also reduce leakages, resulting in increased efficiency and power output.
Due to the harsh environment of gas turbine engines, the engine components are preferably oxidation and corrosion resistant. The abradable seals must also be capable of withstanding the erosive environment that exists due to the entrainment of particulates in the air stream flowing through the gas turbine engine, as well as rubs from the blade tips at extremely high velocities. Because nickel alloys are oxidation and corrosion resistant, abradable seals currently used in the field are typically nickel-based and include nickel-based coatings. While the nickel alloys are successfully used in durable abradable seals, the nickel also increases the overall weight of the gas turbine engine. Another concern with using a nickel-based abradable seal is that nickel has a relatively high coefficient of thermal expansion, which may decrease the thermal cycle durability of the gas turbine engine. Consideration must also be given to the effect that the abradable material may have on downstream components of the gas turbine engine once the abradable material has been worn from the seal and is flowing through the gas turbine engine.